Powder coating compositions are dry, free flowing powders applied in fusion coating processes. A fusion coating process is a process in which a powder coating is distributed over a substrate and, when heated, fuses to form a continuous film. The substrate may be heated or unheated when the powder is applied thereto. Heat supplied from the substrate or from an external source, such as an oven, causes the powder to fuse into the continuous film. Known fusion coating processes for application of powder coating compositions to a substrate include electrostatic spraying, fluidized bed coating, and hot flocking.
One potential application for powder coating compositions is for use as primers and topcoats for military applications. Military equipment, such as wheeled and tracked vehicles, weapons, machinery, and other objects are ideally suited for painting with powder coating compositions. Civilian versions of military equipment and non-military objects can also benefit from the surface protection provided by powder coating compositions.
An advantage of powder coating compositions for both military and civilian applications is that such coating compositions can be formulated to provide a durable, protective finish on the coated article. Such finish can be provided at a relatively low cost because all of the coating can be used; any excess coating material can be reclaimed. Reclamation of any excess coating also avoids costs associated with disposal of waste material.
Another reason that powder coating compositions are attractive for use in military and civilian applications alike is that powder coating compositions have low or no volatile organic compound (VOC) content. There is regulatory pressure to reduce VOC content in coatings because VOCs are known to be air pollutants. A disadvantage of liquid-based coating compositions presently utilized as topcoats in military and civilian applications is that such coating compositions all require VOC-containing solvent systems. Even water-reducible plural-component coating compositions can include from 1.8 to 3.5 pounds VOC content per gallon of coating composition. Powder coating compositions can be formulated to avoid the VOC-containing solvent systems of liquid coating compositions and offer an opportunity to avoid the VOC content associated with liquid coating compositions.
Any powder coating composition contemplated for use as a military coating must presently comply with U.S. Department of Defense Military Specification MIL-PRF-32348 entitled Powder Coating, Camouflage Chemical Agent Resistant Systems (“MIL-PRF-32348”) which is incorporated herein by reference in its entirety. At present, MIL-PRF-32348 Type III covers powder coatings for use on metallic substrates. MIL-PRF-32348 Type III requires that the powder coating must be a chemical-agent-resistant coating. Such chemical-agent-resistant coatings are referred to by the acronym “CARC”. Representative chemical agents against which the coating must be resistant include GD and HD agents. In addition, the powder coating must be free of VOCs, and must be compliant with stringent requirements for color and reflectance, hiding power, specular gloss, infrared reflectance, and other properties and capabilities.
Various powder coating compositions have been developed in an effort to meet MIL-PRF-32348. However, none of these powder coatings have been developed for use as topcoats (i.e., paints) for coating a broad range of objects including vehicles (e.g., wheeled and tracked vehicles, aircraft, ships, etc.), machinery, equipment, buildings, and other structures and articles. There are presently no powder coating compositions for use as a topcoat or primer which are compliant with MIL-PRF-32348 Type III.
Existing powder coating compositions fail to comply with MIL-PRF-32348 Type III because such coating compositions do not provide a finished-form film which includes the combination of weather-resistance, resistant to adsorption of chemical agents, and a surface appearance with the requisite low gloss and sheen. In a topcoat or camouflage coating, it is particularly important that the surface have a matte-appearance with low levels of gloss and sheen because light reflectance, or glare, from the coating surface can result in unwanted detection of the coated article. Avoidance of detection of the coated article is particularly important in military applications involving motor vehicles, aircraft, weapons, and other devices.
In general, three different approaches have been used to control gloss and sheen in existing powder coating compositions. These three approaches have included use of fillers, waxes, and differential cure techniques. While generally useful at controlling light reflection, each approach has certain disadvantages, making them unsuitable for compliance with MIL-PRF-32348 Type III.
Fillers, such as barium sulfate or glass beads, have been utilized to reduce gloss and sheen. Such fillers limit gloss and sheen by reducing the microscopic smoothness of the finished-form coating, thereby scattering light and reducing light reflectance. While fillers are effective at controlling gloss and sheen, fillers have certain disadvantages. Since the filler level necessary to achieve the low gloss and sheen reaches or exceeds the critical pigment volume concentration (CPVC) of the coating, flow is restricted, resistance to environmental exposure is negatively affected, chemical-agent resistance is negatively affected, and interference with flow during spray application occurs because of the high specific gravity of the filler.
Hydrocarbon and fluorocarbon waxes are also used to reduce the gloss and sheen of powder coatings. These waxes limit gloss by forming a surface layer with reduced gloss and sheen. The surface layer is formed when the wax melts during heating of the powder coating on the article and migrates to the coating/air interface where the surface layer is formed. A disadvantage of waxes is that the wax softens the coating surface thereby reducing resistance of the finished-form film to marring, staining, and chemical-agent attack.
Differential cure involves the use of incompatible resins, meaning that the resins have different structures or are catalyzed with different catalysts. Differential cure is particularly effective with powder coating compositions including polyester and acrylic blends. Upon incomplete molecular mixing, such as is typically encountered in a powder coating extruder, these differential-cure systems result in the development of zones of varying shrinkage or varying surface tension on the coating surface during curing. This yields a microscopically-rough surface layer which scatters light and reduces light reflectance. A shortcoming of differential cure is that the incompatible resins may negatively affect the properties of the finished-form film such as impact resistance, marring, staining, resistance to chemical-agent attack, flexibility, and tensile strength.
It would be an improvement in the art to provide powder coating compositions which could be formulated to provide a durable, weather-resistant finished-form film surface with a matte-appearance, which could be formulated in a more environmentally-friendly manner, which could be formulated for compliance with current military specifications including chemical-agent resistance, and which would be economical and easy to apply using conventional powder coating techniques.